Response of the frog skin to steady-state voltage clamping. II. The active pathway

Interpretation of steady-state current-voltage curves: consequences and implications of current subtraction in transport studies

A problem often confronted in analyses of charge-carrying transport processes in vivo lies in identifying porter-specific component currents and their dependence on membrane potential. Frequently, current-voltage (I-V)--or more precisely, difference-current-voltage (dI-V)--relations, both for primary and for secondary transport processes, have been extracted from the overall membrane current-voltage profiles by subtracting currents measured before and after experimental manipulations expected to alter the porter characteristics only. This paper examines the consequences of current subtraction within the context of a generalized kinetic carrier model for Class I transport mechanisms (U.-P. Hansen, D. Gradmann, D. Sanders and C.L. Slayman, 1981, J. Membrane Biol. 63:165-190). Attention is focused primarily on dI-V profiles associated with ion-driven secondary transport for which external solute concentrations usually serve as the experimental variable, but precisely analogous results and the same conclusions are indicated in relation to studies of primary electrogenesis. The model comprises a single transport loop linking n (3 or more) discrete states of a carrier 'molecule.' State transitions include one membrane charge-transport step and one solute-binding step. Fundamental properties of dI-V relations are derived analytically for all n-state formulations by analogy to common experimental designs. Additional features are revealed through analysis of a "reduced" 2-state empirical form, and numerical examples, computed using this and a "minimum" 4-state formulation, illustrate dI-V curves under principle limiting conditions. Class I models generate a wide range of dI-V profiles which can accommodate essentially all of the data now extant for primary and secondary transport systems, including difference current relations showing regions of negative slope conductance. The particular features exhibited by the curves depend on the relative magnitudes and orderings of reaction rate constants within the transport loop. Two distinct classes of dI-V curves result which reflect the relative rates of membrane charge transit and carrier recycling steps. Also evident in difference current relations are contributions from 'hidden' carrier states not directly associated with charge translocation in circumstances which can give rise to observations of counterflow or exchange diffusion. Conductance-voltage relations provide a semi-quantitative means to obtaining pairs of empirical rate parameters.(ABSTRACT TRUNCATED AT 400 WORDS)

Phenamil: an irreversible inhibitor of sodium channels in the toad urinary bladder

Several new amiloride analogues and two reported photoaffinity analogues were tested for irreversible inhibition of short-circuit current, Isc, in toad bladder. Bromoamiloride, a photoaffinity analogue, induced 40% irreversible inhibition at 500 microM after irradiation with ultraviolet light greater than or equal to 320 nm. Iodoamiloride caused no irreversible inhibition. Of the new analogues tested, only 3,5-diamino-6-chloro-N-[(phenylamino) amino-methylene] pyrazinecarboxamide, phenamil, irreversibly inhibited Isc at concentrations of 0.05 to 5 microM when added to the mucosal solution. Irreversible inhibition of Isc by phenamil may be attributed to specific blockage of the mucosal sodium channels, which depended on: 1) time of exposure; 2) mucosal pH; 3) mucosal sodium concentration. For example, 5 microM phenamil irreversibly inhibited Isc by 38% in 103 mM Na at pH 8.6 and nearly 75% in 30 mM Na at pH 6.4 after a 40-min exposure. Irreversible inhibition occurred in two phases with time constants of less than or equal to 10 min and approximately 140 min. Due to its irreversible nature, phenamil may be used to measure channel density.

Intracellular electrolyte concentrations in the frog skin epithelium: effect of vasopressin and dependence on the Na concentration in the bathing media

The intracellular electrolyte concentrations of the frog skin epithelium have been determined in thin freeze-dried cryosections using the technique of electron microprobe analysis. Stimulation of the transepithelial Na transport by arginine vasopressin (AVP) resulted in a marked increase in the Na concentration and a reciprocal drop in the K concentration in all epithelial cell layers. The effects of AVP were cancelled by addition of amiloride. It is concluded from these results that the primary mechanism by which AVP stimulates transepithelial Na transport is an increase in the Na permeability of the apical membrane. However, also some evidence has been obtained for an additional stimulatory effect of AVP on the Na pump. In mitochondria-rich cells and in gland cells no significant concentration changes were detected, supporting the view that these cells do not share in transepithelial Na transport. Furthermore, the dependence of the intracellular electrolyte concentrations upon the Na concentration in the outer and inner bathing solution was evaluated. Both in control and AVP-stimulated skins the intracellular Na concentration showed saturation already at low external Na concentrations, indicating that the self-inhibition of transepithelial Na transport is due to a reduction of the permeability of the apical membrane. After lowering the Na concentration in the internal bath frequently a Na increase in the outermost and a drop in the deeper epithelial layers was observed. It is concluded that partial uncoupling of the transport syncytium occurs, which may explain the inhibition of the transepithelial Na transport and blunting of the AVP response under this condition.

Surface potentials and sodium entry in frog skin epithelium

1. The effects which alterations in the surface potential of the apical membrane of isolated Rana catesbeiana skin have on Na entry were examined. 2. Changes in the external ionic strength have little effect upon the rate of Na transport across the frog skin epithelium. 3. Uranyl ion (UO2(2+), 2.5 mM) induces a +145 mV change in the surface potential of phosphatidylserine monolayers, and a +60mV change in the surface potential of monolayers made from phosphatidylcholine. 4. UO2(2+) inhibits the short-circuit current (Isc) by a maximum of 20% in R. catesbeiana skin, while stimulating Isc by 40% in R. temporaria skin. Neither Isc stimulation nor inhibition by UO2(2+) can be seen in the presence of 10(-4) M-amiloride. 5. From points 1 and 2 above, we conclude that the surface charge density in the neighbourhood of the Na-selective entry site located in the apical membrane is small (greater than 1e-/600 A2). The results obtained using UO2(2+) suggest that Na entry is not affected by changes in the membrane surface potential.

Interpretation of current-voltage relationships for "active" ion transport systems: I. Steady-state reaction-kinetic analysis of class-I mechanisms

This paper develops a simple reaction-kinetic model to describe electrogenic pumping and co- (or counter-) transport of ions. It uses the standard steady-state approach for cyclic enzyme- or carrier-mediated transport, but does not assume rate-limitation by any particular reaction step. Voltage-dependence is introduced, after the suggestion of Läuger and Stark (Biochim. Biophys. Acta 211:458-466, 1970), via a symmetric Eyring barrier, in which the charge-transit reaction constants are written as k12 = ko12 exp(zF delta psi/2RT) and k21 = ko21 exp(-zF delta psi/2RT). For interpretation of current-voltage relationships, all voltage-independent reaction steps are lumped together, so the model in its simplest form can be described as a pseudo-2-state model. It is characterized by the two voltage-dependent reaction constants, two lumped voltage-independent reaction constants (k12, k21), and two reserve factors (ri, ro) which formally take account of carrier states that are indistinguishable in the current-voltage (I-V) analysis. The model generates a wide range of I-V relationships, depending on the relative magnitudes of the four reaction constants, sufficient to describe essentially all I-V datas now available on "active" ion-transport systems. Algebraic and numerical analysis of the reserve factors, by means of expanded pseudo-3-, 4-, and 5-state models, shows them to be bounded and not large for most combinations of reaction constants in the lumped pathway. The most important exception to this rule occurs when carrier decharging immediately follows charge transit of the membrane and is very fast relative to other constituent voltage-independent reactions. Such a circumstance generates kinetic equivalence of chemical and electrical gradients, thus providing a consistent definition of ion-motive forces (e.g., proton-motive force, PMF). With appropriate restrictions, it also yields both linear and log-linear relationships between net transport velocity and either membrane potential or PMF. The model thus accommodates many known properties of proton-transport systems, particularly as observed in "chemiosmotic" or energy-coupling membranes.

Kinetics of ionic transport across frog skin: two concentration-dependent processes

Sodium and chloride influxes across the nonshort-circuited isolated skin of Rana esculenta were measured at widely varying external ionic concentrations. The curve describing sodium transport has two Michaelis-Menten components linked at an inflection point occurring at an external sodium concentration of about 7 meq. Chloride transport can also be represented by two saturating components. A possible explanation of these kinetics is discussed. At sodium concentrations lower than 4 meq it is possible to define a component of the sodium transport mechanism as having a high affinity for sodium and which is independent of the nature of the external anion. A high affinity for chloride of the chloride transport system functioning at low external concentrations is also found but is significantly different from that of sodium. These systems show the physiological characteristics of the countertransports (Na est(+)/H int(+); Cl ext(-)/HCO 3int(-)) functioning at low external concentrations. At external concentrations higher than 4 meq a low affinity transporting system in which chloride and sodium are linked superimpose on the high affinity compoents. The physiological significance of these results is discussed.

Ba2+-induced conductance fluctuations of spontaneously fluctuating K+ channels in the apical membrane of frog skin (Rana temporaria)

We studied the influence of mucosal Ba2+ ions on the recently described (Zeiske & Van Driessche, 1979a, J. Membrane Biol. 47:77) transepithelial, mucosa towards serosa directed K+ transport in the skin of Rana temporaria. The transport parameters G (conductance), PD (potential difference), Isc (short-circuit current, "K+ current"), as well as the noise of Isc were recorded. Addition of millimolar concentrations of Ba/+ to the mucosal K+-containing solution resulted in a sudden but quickly reversible drop in Isc. G and Isc decreased continuously with increasing Ba2+ concentration, (Ba2+)o. The apparent Michaelis constant of the inhibition by Ba2+ lies within the range 40-80 microM. The apical membrane seems to remain permselective for K+ up to 500 microM (Ba2+)o. Higher (Ba2+)o, however, appears to induce a shunt (PD falls, G increases). This finding made an accurate determination of the nature of the inhibition difficult but our results tend to suggest a K+-channel block by K+-Ba2+ competition. In the presence of Ba2+, the power spectrum of the K+ current shows a second Lorentzian component in the low-frequency range, in addition to the high-frequency Lorentzian caused by spontaneous K+-channel fluctuations (Van Driessche & Zeiske, 1980). Both Lorentzian components are only present with mucosal K+ and can be depressed by addition of Cs+ ions, thus indicating that Ba2+ ions induce K+-channel fluctuations. The dependence of the parameters of the induced Lorentzian on (Ba2+)o shows arise in the plateau values to a maximum around 60 microM (Ba2+)o, followed by a sharp and progressive decrease to very low values. The corner frequency which reflects the rate of the Ba2+-induced fluctuations, however, increases quasi-linearly up to 1 mM (Ba2+)o with a tendency to saturate at higher (Ba2+)o. Based on a three-state model for the K+ channel (having one open state, one closed by the spontaneous fluctuation and one blocked by Ba2+) computer calculations compared favorably with our results. The effect of Ba2+ could be explained by assuming reversible binding at the outer side of the apical K+ channel, thereby blocking the open channel in ;competition with K+. The association-dissociation of Ba2+ at its receptor site is thought to cause a chopping of the K+ current, resulting in modulated current fluctuations.

Protocol-dependence of equivalent circuit parameters of toad urinary bladder

Determinations o- current-voltage relationships are widely employed in the characterization of epithelial sodium transport. In order to determine the protocol dependence of transport parameters in the toad urinary bladder, studies were carried out in the presence and absence of amiloride, an inhibitor of active sodium transport. With symmetric positive and negative perturbations of the transepithelial electrical potential difference delta psi (0 leads to +/- 100 mV) for 30 sec, the amiloride-sensitive current-voltage (ia-delta psi) relationship was near linear over the range -75 leads to +100 mV, indicating constancy of the conductance ka and the apparent electromotive force "ENa", lumped parameters of the standard electrical equivalent circuit model of the active transport system. With a reverse protocol (+/- 100 leads to 0 mV) or 15 min perturbations the ia-delta psi relationships were highly nonlinear. Nonlinearity reflected voltage dependence of parameters: perturbations that increased active transport decreased "ENa" and increased ka, as evaluated from 10 sec perturbations of delta psi; slowing of active transport produced the converse changes. These effects are usefully analyzed in both quasi-steady states and true steady states by means of a detailed equivalent circuit incorporating the significant ionic currents across each plasma membrane. Precise understanding of the significance of ka and "ENa" will require characterization of the partial ionic conductances on perturbation of delta psi.

K+-permeability of the outer border of the frog skin (R. temporaria)

Skins from Rana temporaria, investigated with microelectrode techniques in the absence of Na uptake across the outer border (Na-free epithelial solution or amiloride), were found to be permeable to K+ at the apical membrane in 10-20% of the experiments. Full development of the K+-permeable state requires the absence of Na+ uptake for certain periods of time, which suggests that the K+-permeability of the apical membrane is higher at lower intracellular [Na]. The addition of Ba++ reduces the K+-permeability of the apical membrane. These skins may provide a model for the study of transcellular K+ movements.

On the cross-reactivity of amiloride and 2,4,6 triaminopyrimidine (TAP) for the cellular entry and tight junctional cation permeation pathways in epithelia

2,4,6 Triaminopyrimidine (TAP) has been previously shown to inhibit the passive tight junctional cation permeation pathway in various "leaky" epithelia. Amiloride has been shown to be an effective inhibitor of the cation cellular entry pathway in "tight" epithelia. In this paper we demonstrate that TAP and amiloride at appropriate concentrations are able to block either of these epithelial cation permeation pathways. TAP was found to block the Na entry pathway in frog skin with the following characteristics: it (1) inhibits from the external solution only, (2) is completely reversible, (3) increases the transepithelial resistance, (4) is active in the monoprotonated form, (5) is noncompetitive with Na, (6) displays saturation kinetics which obey a simple kinetic model (KI = 1 X 10(-3) M), (7) is independent of external calcium, (8) is dependent on external buffering capacity, and (9) is competitive with amiloride. Amiloride inhibition of the junctional permeation in gallbladder had the following characteristics: it (1) increases the transepithelial resistance, (2) decreases cation conductance without affecting the anion conductance, (3) displays saturation kinetics which obey a simple kinetic model (KI = 1 X 10(-3) M), and (4) possesses inhibitory activity in both its protonated and unprotonated form. These results not only indicate that a similar inhibitory site may exist in both of these cation permeation pathways, but also provide information on the chemical nature and possible location of these inhibitory sites.

Transient potassium fluxes in toad skin

Experiments were carried out in the isolated short-circuited skin of the toad Bufo marinus ictericus. 42K influx and efflux experiments were carried out with skins bathed on both sides by NaCl-Ringer's solution. Those fluxes showed very similar kinetics of equilibration with time and the results could be fitted by equations of a model of two intraepithelial compartments and the bathing solutions. In the steady state K influx is 3.99 +/- 0.36 nmol cm-2 hr-1 (n = 7) and efflux 3.62 +/- 0.38 nmol cm-2 hr-1 (n = 7) and are not statistically different, indicating that no net K flux is present across the epithelium. Different kinds of perturbations affecting the rates of 42K discharge into the bathing solutions were studied. Immediately after addition of amiloride (10(-4) M) to the outer solution, a sharp decline is observed in the rate of 42K discharge into the bathing solution, JK21, which falls from 3.62 +/- 0.38 nmol cm-2 hr-1 to 2.02 +/- 0.04 nmol cm-2 hr-1 (n = 7) 2 min after addition of the drug, followed by a partial recuperation with time. A complete Na by K substitution in the outer bathing solution induces a prompt and marked decline in JK21 which is similar to that induced by amiloride. Increase in the outer bathing solution Na concentration from zero Na concentration induces a nonlinear increase in JK21 and a linear relationship was observed between JK21 and short-circuit current in the range of 0 to 115 mM external Na concentration. The decline in JK21 induced by amiloride or by lowering external Na concentration was interpreted as being caused by electrical hyperpolarization of the external barrier of the epithelium induced by these procedures. Depolarization of the epithelial barriers by inner Na by K substitution in the short-circuited state (when the potential barriers are equal) drastically interfere with the rate of 42K discharge from the epithelium into the bathing solutions. Thus, transient increases are observed both in the rate of 42K discharge to the outer and to the inner bathing solutions upon depolarization of the barriers. These results indicate that at least the most important component of transepithelial K unidirectional fluxes goes through a transcellular route with a negligible paracellular component. Addition of ouabain (10(-3) M) to the inner bathing solution induces a transient rise in the rate of 42K discharge to the outer bathing solution with a peak on the order of 200% of the stationary value previous to the action of the inhibitor, followed by a return to new stationary values not statistically different from those observed previously to the effect of ouabain. The behavior of JK21 upon the effect of ouabain, as suggested by comparison with predictions from computer simulation, strongly supports the notion of a rheogenic Na pump in the inner barrier of the epithelium against the notion of a nonrheogenic 1:1 Na--K pump.

Saturable K+ pathway across the outer border of frog skin (rana temporaria): kinetics and inhibition by Cs+ and other cations

The reaction of abdominal skins of the frog species Rana temporaria on mucosal K+-containing solutions was studied in an Ussing-type chamber by recording transepithelial potential difference (PD), short-circuit current (SCC) and conductance (G). With Na-Ringer's as serosal medium, a linear correlation between PD and the logarithm of the mucosal K+-concentration ([K]o) was obtained. The K+-dependent SCC saturated with increasing [K]o, and could quickly and reversibly be depressed by addition of Rb+, Cs+, and H+. Li+, Na+, and NH4+ did not influence K+ current. A large scatter was obtained for kinetic parameters like the slope of the PD-log[K]o-line (18--36.5 mV/decade), the apparent Michaelis constant (13--200 mM), and the maximal current of the saturable SCC (6--50 microa . cm-2), as well as for the degree of inhibition by Cs+ ions. This seemed to be caused by a time-dependent change during long time exposure to high [K]o (more than 30 sec), thereby inducing a selectivity loss of K+-transporting structures, together with an increase in SCC and G and a decrease in PD. Short time exposure to K+-containing solutions showed a competitive inhibition of K+ current by Cs+ ions, and a Michaelis constant of 6.6 mM for the inhibitory action of Cs+. Proton titration resulted in a decrease of K+ current at pH less than 3. An acidic membrane component (apparent dissociation constant 2.5 x 10(-3) M) is virtually controlling K+ transfer. Reducing the transepithelial K+-concentration gradient by raising the serosal potassium concentration was accompanied by the disappearance of SCC and PD.

On the mechanism of the amiloride-sodium entry site interaction in anuran skin epithelia

The steady-state transport kinetics of the interaction between external sodium and the diuretic drug, amiloride, was studied in isolated anuran skin epithelia. We also investigated the effect of calcium on the amiloride-induced inhibition of short-circuit current (Isc) in these epithelial preparations. The major conclusions of this study are: (a) amiloride is a noncompetitive inhibitor of Na entry in bullfrog and grassfrog skin, but displays mixed inhibition in R. temporaria and the toad. A hypothesis which states that the interaction sites for amiloride and Na on the putative entry protein are spatially distinct in all of these species is proposed. (b) The stoichiometry of interaction between amiloride and the Na entry mechanism is not necessarily one-to-one. (c) The external Ca requirement for the inhibitory effect of amiloride is not absolute. Amiloride, at all concentrations, is equally effective in inhibiting Isc of bullfrog skin independently from the presence or absence of external Ca.

Binding of [3H]ouabain to split frog skin: the role of the Na,K-ATPase in the generation of short circuit current

The binding of [3H]ouabain to the serosal side was studied in a chambered preparation of frog skin, free of connective tissue, while the short circuit (Isc) was concurrently monitored. Both ouabain binding and Isc inhibition proceeded as hyperbolic functions of time. A plot of the number of ouabain molecules bound vs. the corresponding values of Isc inhibition (percent) yielded a straight line, yet showed that one-third of the binding occurred before any inhibition of Isc. Upon separation of the skins into two groups based upon initial Isc(Isci)--high, greater than 20 microamperemeter/cm2 and low, less than 10 microamperemeter/cm2, we observed two distinct populations. The high Isci skins bound very little ouabain before inhibition of Isc whereas low Isci skins bound one-half of the total number of sites before exhibiting any inhibition of Isc. These observations strongly suggest that (a) the Na,K-ATPase is directly involved in the generation of Isc, and (b) at low Isc, inhibition of some pumps by ouabain causes a "recruitment" of other pumps to increase their turnover rate and maintain Isc relatively unaffected. In addition, the binding of ouabain also displayed various characteristics that were consistent with known properties of the Na,K-ATPase: (a) increased intracellular K/Na concentrations, whether achieved through the addition of amiloride or removal of Na from the outside medium, led to a significant decrease in ouabain binding rate relative to paired controls; and (b) ouabain binding, either with normal or decreased intracellular Na, was significantly reduced in the presence of elevated K in the serosal bathing medium. Finally, the number of ouabain molecules bound to the frog skins was not correlated with their initial Isc values, indicating that the spontaneous skin-to-skin variation in Isc was not related to the number of functional pump sites but, rather, to their turnover rate.

Current-voltage relationships for the plasma membrane and its principal electrogenic pump in Neurospora crassa: I. Steady-state conditions

The nonlinear membrane current-voltage relationship (I-V curve) for intact hyphae of Neurospora crassa has been determined by means of a 3-electrode voltage-clamp technique, plus "quasi-linear" cable theory. Under normal conditions of growth and respiration, the membrane I-V curve is best described as a parabolic segment convex in the direction of depolarizing current. At the average resting potential of - 174 mV, the membrane conductance is approximately 190 micronhos/cm2; conductance increase to approximately 240 micronhos/cm2 at -300 mV, and decreases to approximately 130 micronhos/cm2 at 0 mV. Irreversible membrane breakdown occurs at potentials beyond this range. Inhibition of the primary electrogenic pump in Neurospora by ATP withdrawal (with 1 mM KCN) depolarizes the membrane to the range of -40 to -70 mV and reduces the slope of the I-V curve by a fixed scaling factor of approximately 0.8. For wild-type Neurospora, compared under control conditions and during steady-state inhibition by cyanide, the I-V difference curve--presumed to define the current-voltage curve for the electrogenic pump--is a saturation function with maximal current of approximately 20 muA/cm2, a half saturation potential near -300 mV, and a projected reversal potential of ca. -400 mV. This value is close to the maximal free energy available to the pump from ATP hydrolysis, so that pump stoichiometry must be close to 1 H+ extruded:1 ATP split. The time-courses of change in membrane potential and resistance with cyanide are compatible with the steady-state I-V curves, under the assumption the cyanide has no major effects other than ATP withdrawal. Other inhibitors, uncouplers, and lowered temperature all have more complicated effects. The detailed temporal analysis of voltage-clamp data showed three time-constants in the clamping currents: one of 10 msec, for charging the membrane capacitance (0.9 muF/cm/2); a second of 50-75 msec; and a third of 20-30 sec, perhaps representing changes of intracellular composition.

Transients in toad skin: short circuit current and ionic fluxes related to inner sodium substitution by monovalent cations

When the Na electrochemical potential difference across the skin (delta muNa) is altered by perturbing the transmembrane electrical potential difference or the external Na concentration, effects on transport and associated oxygen consumption can be described by the formalism of linear nonequilibrium thermodynamics (Vieira, Caplan & Essig, 1972, J. Gen. Physiol. 59:77; Danisi & Lacaz-Vieira, 1974, J. Gen. Physiol. 64:372; Procópio and Lacaz-Vieira, 1977, J. Membrane Biol. 35:219). We now show that with modifications of delta muNa by substitution of Li or choline for Na in the inner bathing solution, this formalism is no longer applicable. Inner Na by K substitution ((Na X K)i) causes profound alterations in short-circuit current (SCC), JinNa, K efflux (JeffK) and PD. SCC drops transiently after (Na X K)i in Cl and in SO4 media, increasing subsequently. In Cl medium, following the initial transient, there is a late decline in SCC toward a steady state. The rate of SCC decline in Cl medium is more pronounced than that observed in SO4 medium. (Na X K)i causes a transient increase in JinNa with a peak synchronous to the minimum in SCC, both in Cl and in SO4 media. This was interpreted as due to depolarization of the inner membrane. In SO4 medium, following the peak observed after (Na X K)i, JimNa drops, to increase again toward a steady state in which SCC and JinNa are not statistically different, resembling the control condition before (Na X K)i. In Cl medium, however, the JinNa steady state is approximately 100% higher than SCC. This difference is due to an important K efflux (JeffK), which builds up progressively after the substitution. The apparent K permeability [JeffK/(Ki)] is of comparable magnitude in Cl and in SO4 media before (Na X K)i and also in SO4 medium after (Na X K)i. However, in Cl medium, after (Na X K)i, the apparent K permeability increases one order of magnitude as compared to the control condition before the ionic substitution. In Cl medium, the high levels of JinNa and of Jeff(K) observed in the steady state after (Na X K)i were interpreted as being a consequence of cell swelling. SCC and PD follow very different temporal patterns after (Na X K)i which are characterized by transients in SCC and a simple fall in PD. Reasons for these differences are discussed.

Irreversible inhibition of sodium entry sites in frog skin by a photosensitive amiloride analog

A photosensitive binding reaction is described in which an analog of amiloride is bound to sites that control sodium entry into frog skin. This reaction results in irreversible inhibition of net sodium transport.

Nonhormonal mechanisms for the regulation of transepithelial sodium transport: the roles of surface potential and cell calcium

An attempt to define the main categories of regulatory mechanisms of transepithelial sodium transport across tight epithelia is presented. In particular, evidence suggesting two types of mechanisms, changes in surface potential and the level of cell Ca, are described in greater detail. We have measured the effects of conditions that affect surface potential on the transepithelial sodium transport. Those conditions that increase the screening of negative charge and therefore depolarize the outer membrane are expected to have effects homologous to a depolarization caused by external current. Indeed, when the composition of the outside solution was modified by (i) increasing ionic strength, (ii) adding polyvalent cations (La+++, Co++, Ni++, Cd++), or (iii) lowering pH, an increase in active Na transport was detected. Moreover, the presence of small concentrations of polyvalent cations which screen surface charge, markedly dampens or even eliminates the effects of pH or ionic strength on Na transport. These findings provide additional support for the notion that a potential-sensitive component regulates Na movements across the apical membrane of the frog skin, and offer a framework to understand the effects of numerous cationic agents on transepithelial transport that hitherto remain unexplained. With respect to the role of intracellular Ca we have found that procedures that increase cell Ca, like removal of sodium in the basal solution or addition of ionophore A23187, reduce transepithelial Na transport. Moreover, conditions that block the increase in cell Ca prevent the inhibition of transport. These observations suggest that the level of intracellular Ca may determine the rate of transepithelial Na transport.

Effects of electrical gradients on volume flows across gall bladder epithelium

A volumetric method has been developed which permits continuous registration of volume flows across epithelial tissues. The method was applied to volume flow measurements across rabbit gall bladder epithelium. The rate of fluid reabsorption measured in this way was twice as high as previously observed in sac preparations of the gall bladder. This is probably due to better aeration and stirring of the mucosal solution. It was demonstrated that electrical gradients across the gall bladder induced volume flows towards the negative electrode. In non-transporting bladders volume flows were linearly related with current between 300 and 900 muA in both directions. However, volume flow rates were three times higher from mucosa to serosa than in the opposite direction. From the magnitude of polarization potentials, observed after switching off the current, the conclusion was reached that all of the current-induced volume flow is an osmotic flow due to salt polarization in the unstirred layers of the tissue. By implication, so-called streaming potentials observed during osmotic flows reflect solely polarization effects. In actively transporting gall bladders a 200 muA current increased or decreased the flow rate twice as much as expected from polarization effects alone. Therefore passage of current interfered directly with the active transport mechanism of gall bladder epithelium.

The electrical potential profile of gallbladder epithelium

In this study the relative ionic permeabilities of the cell membranes of Necturus gallbladder epithelium have been determined by means of simultaneous measurement of transmural and transmucosal membrane potential differences (PD) and by ionic substitution experiments with sodium, potassium and chloride ions. It is shown that the mucosal membrane is permeable to sodium and to potassium ions. The baso-lateral membrane PD is only sensitive to potassium ions. In both membranes chloride conductance is negligible or absent. The ratio of the resistances of the mucosal and baso-lateral membranes, RM/RS, increases upon reducing the sodium concentration in the mucosal solution. The same ratio decreases when sodium is replaced by potassium which implies a greater potassium than sodium conductance in the mucosal membrane. The relative permeability of the shunt for potassium, sodium and chloride ions is: PK/PNa/PCl=1.81:1.00:0.32. From the results obtained in this study a value for the PK/PNa ratio of the mucosal membrane could be evaluated. This ratio is 2.7. From the same data the magnitude of the electromotive forces generated across the cell membranes could be calculated. The EMF's are -15mV across the mucosal membrane and -81mV across the baso-lateral one. Due to the presence of the low resistance shunt the transmucosal membrane PD is -53.2mV (cell inside negative) and the transmural PD is +2.6mV (serosal side positive). The change in potential profile brought about by the low resistance shunt favors passive entry of Na ions into the cell across the mucosal membrane. Calculations show that this passive Na influx is maximally 64% of the net Na flux estimated from fluid transport measurements. The C-1 conductive of the baso-lateral membrane is too small to allow electrogenic coupling of C1 with Na transport across this membrane. Experiments with rabbit gallbladder epithelium indicate that the membrane properties in this tissue are qualitatively similar to those of Necturus gallbladder epithelium.

Some features of hydrogen (ion) secretion by the frog skin

We have studied the movements of H+ from the in vitro frog skin into the outside solution because it has been suggested that the movement of sodium from the outside solution into the skin may result from the forced exchange of Na+ by H+. Our main observations can be summarized as follows: (a) Hydrogen moves from the skin into the outside solution at a rate of 0.04 muequiv-cm-2-h-1 while Na+ influx had a value of 0.49 muequiv-cm-2-h-1. (b) The rate of H+ secretion is not significantly affected by substituting the Na+ in the outside solution by K+ nor by inhibiting Na+ influx with amiloride (5-10(-5) M). (c) Acetazolamide (5-10(-3) M) blocked H+ secretion without altering the potential difference across the skin. (d) The rate of H+ production is not underestimated because it may have been neutralized by HCO3- secreted into the outside solution in exchange for Cl-. Substituting all the Cl- by SO4(2-) in the outside solutions does not result in an increase in the rate of H+ production. (e) The steady-state rate of H+ secretion is not affected by large changes in electrochemical potential gradients for H+. Neither abolishing the potential difference across the skin nor a 10-fold change in H+ concentration in the outside solution affected significantly the steady-state rate of H+ secretion. (f) The H+ secretion was abolished by the metabolic inhibitors dinitrophenol (1-10(-4) M) and Antimycin A (1.5-10(-6) M) which also markedly reduced the potential difference across the skin. Observations (a), (b), and (c) suggest that H+ and Na+ movements across the outer border of the isolated frog skin are not coupled. The ratio of Na+ to H+ movements is very different from unity and Na+ movements can be abolished without any effects on H+ secretion and conversely H+ movements can be abolished without interruption of Na+ uptake. A second conclusion suggested by these results is that the H+ secretion does not result from movement of H+ following its electrochemical potential gradient since that rate of secretion is not affected by marked changes in either potential or [H+]. Furthermore, the effects of metabolic inhibitors suggest that H+ secretion requires the expenditure of energy by the cell.

Sodium fluxes through the active transport pathway in toad bladder

To assess the active components of sodium flux across toad bladder as a function of transepithelial potential, unidirectional sodium fluxes between identical media were measured before and after adding sufficient ouabain (1.89 X 10(-3)M) to eliminate active transport, while clamping transepithelial potential to 0, 100 or 150 mV. Evidence was adduced that ouabain does not alter passive fluxes, and that fluxes remain constant if ouabain is not added. Hence, the ouabain-inhibitable fluxes represent fluxes through the active path. Results were analyzed by a set of equations, previously shown to describe adequately passive fluxes under electrical gradients in this tissue, here modified by the insertion of E, the potential at which bidirectional sodium fluxes (beta E, and theta E) through the active pathway are equal. According to these equations, beta E and theta E are the logarithmic mean of bidirectional fluxes through the active path at any potential, and the flux ratio in this path is modified by a constant factor Qia, which represents the ratio of the bulk diffusion coefficient to the tracer diffusion coefficient in this pathway. The data are shown to conform closely to these equations. Qia averages 2.54. Hence, serosal-to-mucosal flux vanishes rapidly as potential falls below E. Mean E in these experiments was 158 +/- 1 mV. Thus, linear dependence of net flux in both active and passive pathways on potential is present, even though the sodium fluxes in both paths fail to conform to the Ussing flux ratio equation. Qip less than 1 in the passive path (qualitatively similar to exchange diffusion) and Qia greater than 1 in the active path (as in single file pore diffusion). Both of these features tend to reduce the change in serosal-to-mucosal sodium flux induced by depolarization from spontaneous potential to zero potential ("short-circuiting").

The sodium-transporting compartment of the epithelium of frog skin

1. The abdominal frog skin was mounted between two chambers containing Ringer with 1 mM-Na on the outside and 115 mM-Na on the inside. When the Na concentration of the outer solution ([Na](o)) is instantaneously raised from 1 to 50 mM, the short circuit current (I) increases to a new value in less than a second, and becomes essentially time-independent. Only in a few experiments was it observed to increase further, although at a much slower rate.2. At a time t after this increase, the addition of 10(-4)M amiloride to the outer solution produces an exponential decrease of I. The area under this exponential curve is generally taken to reflect the existence of a Na- transporting compartment (NaTC).3. The amount of Na represented by NaTC is a function of t: it increases from 1.7 x 10(-9) mole. cm(-2), at t = 10 sec, to 22.8 x 10(-9) mole. cm(-2) at t = 10 min.4. In view of the fact that (a) I is not a function of the size of the ;NaTC' and (b) that whereas I reaches a steady value in a fraction of a second the size of NaTC keeps increasing for minutes, it is proposed that the ;NaTC' represents an amount of Na which is not located along the main route of transepithelial transport.5. On the assumption that the NaTC is located in a cellular compartment and that, in order to accumulate in this compartment Na should be accompanied by a permeable anion, a series of experiments were performed with Ringer in which Cl(-) was replaced by gluconate. It was observed as expected, that NaTC in gluconate is 164 times smaller than in Cl(-), but I only decreases to one half its value in Cl(-) Ringer.